Low-sulfur article having a platinum aluminide protective layer and its preparation

ABSTRACT

A coated article is prepared by furnishing a nickel-base article substrate having a free sulfur content of more than 0 but less than about 1 part per million by weight. A protective layer is formed at a surface of the article substrate. The protective layer includes a platinum aluminide diffusion coating. The protective layer may be substantially yttrium-free, or have a controlled amount of yttrium. A ceramic layer may overlie the protective layer.

This application is a continuation of Ser. No. 10/013,878, filed Dec.11, 2001, now U.S. Pat. No. 6,797,408, which itself is a continuation ofapplication Ser. No. 09/328,574, filed Jun. 9, 1999, now U.S. Pat. No.6,333,121, which is a continuation-in-part of application Ser. No.09/149,018, filed Sep. 8, 1998, now U.S. Pat. No. 6,551,423. The '878application is also a continuation-in-part of Ser. No. 08/398,259, filedMar. 3, 1995, now U.S. Pat. No. 6,656,605, which in turn is acontinuation-in-part of application Ser. No. 07/960,494, filed Oct. 13,1992, now U.S. Pat. No. 5,538,796.

BACKGROUND OF THE INVENTION

This application relates to coated articles, and, more particularly, toa superalloy article having a metallic overlay protective coating.

In an aircraft gas turbine (jet) engine, air is drawn into the front ofthe engine, compressed by a shaft-mounted compressor, and mixed withfuel. The mixture is burned, and the hot exhaust gases are passedthrough a turbine mounted on the same shaft. The flow of combustion gasturns the turbine, which turns the shaft and provides power to thecompressor. The hot exhaust gases flow from the back of the engine,driving it and the aircraft forwardly.

The hotter the combustion and exhaust gases, the more efficient is theoperation of the jet engine. There is thus an incentive to raise thecombustion and exhaust gas temperatures. However, the maximumtemperature of the combustion gases is normally limited by the materialsused to fabricate the turbine vanes and turbine blades of the turbine.In current engines, the turbine vanes and blades are made ofnickel-based superalloys, and can operate at temperatures of up to1900-2100° F.

Many approaches have been used to increase the operating temperaturelimit of the turbine blades and vanes to their current levels. Thecomposition and processing of the materials themselves have beenimproved, and physical cooling techniques are employed.

In another approach, a protective layer or a ceramic/metal thermalbarrier coating (TBC) system is applied to the turbine blade or turbinevane component, which acts as a substrate. The protective layer with nooverlying ceramic layer (in which case the protective layer is termed as“environmental coating”) is useful in intermediate-temperatureapplications. The currently known protective layers include diffusionaluminides and MCrAlY(X) overlays.

A ceramic thermal barrier coating layer may be applied overlying theprotective layer, to form a thermal barrier coating system (in whichcase the protective layer is termed a “bond coat”). The thermal barriercoating system is useful in higher-temperature applications. The ceramicthermal barrier coating insulates the component from the combustion gas,permitting the combustion gas to be hotter than would otherwise bepossible with the particular material and fabrication process of thesubstrate.

Although these coating systems are operable, there is always the need toachieve further improvements in maximum operating temperatures and timesof coated articles. The present invention fulfills this need, andfurther provides related advantages.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method for preparing a coated articleprotected by a protective layer, and the article itself. The protectivelayer is suitable for use as an environmental coating with no overlyingthermal barrier coating, or as the bond coat for a thermal barriercoating. The elevated-temperature oxidation performance of the coatedarticle is improved over that of conventional coated articles.

A coated article comprises an article substrate having a free sulfurcontent of more than 0 but less than about 1 part per million by weight(ppmw), and a protective layer at a surface of the article substrate.The article is preferably a nickel-base superalloy, in the shape of acomponent of a gas turbine aircraft engine such as a turbine blade orturbine vane. The protective layer comprises a platinum aluminidediffusion coating. A thermal barrier coating layer made of a ceramicsuch as yttria-stabilized zirconia may overlie the protective layer,which is then termed a bond coat. There may instead be no overlyingceramic thermal barrier coating layer, in which case the protectivelayer is termed an environmental coating. Both the protective layer andthe ceramic thermal barrier coating layer, where present, are also lowin sulfur, preferably less than about 1 ppm by weight.

The protective layer may be substantially yttrium free, with less thanabout 10 parts per million. The protective layer may instead contain asubstantial amount of yttrium, typically from about 10 to about 200parts per million, for other applications.

The substrate article with low free sulfur content may be furnished in avariety of ways. The base metal may be selected to have a low freesulfur content. The composition of the base metal may be modified toresult in a low free sulfur content. The composition of the base metalmay be modified to result in a low free sulfur content. In one approach,a sulfur-scavenging element such as hafnium or zirconium is provided inthe base metal in an amount sufficient to reduce the free sulfur contentto less than about 1 part per million (ppm) by weight. In anotherapproach, a conventional high-sulfur base metal can be provided. Thebase metal is contacted to a reducing gas to remove sulfur and reducethe free sulfur to the required low level. For example, the base metalcan be contacted at elevated temperatures to hydrogen or ahydrogen-containing gas that desulfurizes the metal. In yet anotherapproach, the molten base metal may be placed into contact with areactive element such as calcium or magnesium, to react with and reducethe free sulfur content.

In the past, in many instances the sulfur content of the underlyingsubstrate upon which the protective layer is deposited has not beenreported. It may not be concluded from the absence of reporting of thesulfur content that the sulfur content is zero or otherwise less thanabout 1 part per million. Instead, in such situations it may beconcluded that the sulfur content is likely in the typical range ofabout 5 to about 30 parts per million by weight, and that the sulfurcontent was not reported because there was no realization of itssignificance at the time.

The coated article of this type can be used in high-temperatureapplications in severe environments. A preferred application is as a gasturbine blade or vane, but the invention is not so limited. Otherfeatures and advantages of the present invention will be apparent fromthe following more detailed description of the preferred embodiment,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a gas turbine component article;

FIG. 2 is a sectional view through the article of FIG. 1, takengenerally along line 2-2, illustrating a protective layer on the surfaceof the article; and

FIG. 3 is a block diagram of one embodiment of the approach of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a component of an aircraft gas turbine engine such as aturbine blade or turbine vane, and in this case is depicted as a turbineblade 20. Other gas turbine engine components can benefit from thecoating approach of the invention, such as, for example, combustorliners, turbine seals, exhaust nozzles, and shrouds. The turbine blade20 includes an airfoil 22 against which the flow of hot exhaust gas isdirected. The turbine blade 20 is mounted to a turbine disk (not shown)by a dovetail 24 which extends downwardly from the airfoil 22 andengages a slot on the turbine disk. A platform 26 extends longitudinallyoutwardly from the area where the airfoil 22 is joined to the dovetail24. A number of cooling channels may extend through the interior of theairfoil 22, ending in openings 28 in the surface of the airfoil 22. Aflow of cooling air is directed through the cooling channels, to reducethe temperature of the airfoil 22.

FIG. 2 illustrates a protective system 30 deposited upon the turbineblade 20, which thereby acts as a substrate 32. The substrate 32 may beformed of any operable material, but a preferred base metal from whichthe article substrate is formed is a nickel-base superalloy. Anickel-base superalloy had more nickel than any other element. Apreferred nickel-base superalloy is Rene' N5, which has a nominalcomposition in weight percent of 7.5 percent cobalt, 7 percent chromium,6.2 percent aluminum, 6.5 percent tantalum, 5 percent tungsten, 1.5percent molybdenum, 3 percent rhenium, balance nickel. The articlesubstrate is preferably, but not necessarily, directionally solidifiedor single crystal.

The substrate 32 has a free sulfur content of less than about 1 part permillion by weight (ppmw), more preferably less than about 0.5 parts permillion by weight, and most preferably on the order of about 0.2 partsper million by weight. Larger amounts of free sulfur interfere with theadherence of coatings to the surface of the substrate during service.The free sulfur content need meet this requirement only locally, nearthe surface of the substrate 32. Because sulfur diffuses through thesubstrate 32 during elevated temperature exposure, it is, however,preferred that the free sulfur content be below about 1 ppm throughoutat least most of, and most preferably all, the substrate material.

The substrate 32 has a yttrium content of no greater than about 200parts per million by weight. If the substrate has a yttrium content ofmore than about 200 parts per million by weight, it is extremelydifficult to cast, with the result that manufacturing costs areincreased, and the added yttrium increases the cost but has little addedeffect on improved properties. For less demanding applications, thesubstrate has substantially no yttrium, which is herein defined as lessthan about 10 parts per million by weight yttrium. The yttrium in thisrange adds little to the cost of the substrate material, and does notsubstantially increase the difficulty in casting the substrate material.However, in some more demanding applications such as articles exposed tovery high temperatures for extended periods of time, the yttrium ispresent in an amount of from about 10 to about 200 parts per million byweight. The added cost and casting difficulties associated with thishigher yttrium range are offset by the improved performance that isachieved with more than about 10 parts per million by weight yttrium.

A protective layer 34 is present at a free surface 36 of the substrate32. The protective layer 34 is a diffusion platinum aluminide. Thediffusion platinum aluminide of the protective layer 34 of FIG. 2 is ametallic composition containing platinum, aluminum, and the elementspresent in the substrate 32. The diffusion aluminide is formed bydepositing one or more sublayers overlying the surface 36, and theninterdiffusing the deposited sublayers. For example, a sublayercontaining platinum is first deposited upon the free surface 36, andthen a sublayer containing aluminum is deposited over the platinumsublayer at a temperature sufficient that the platinum and aluminumsublayers interdiffuse with each other and with the material of thesubstrate to form a platinum-aluminum coating layer. In a preferredembodiment, the platinum is present in an average amount of from about20 to about 30 weight percent, preferably from about 25 to about 28weight percent, of the protective layer 34, and the aluminum is presentin an average amount of from about 14 to about 25 weight percent,preferably from about 18 to about 22 weight percent, of the protectivelayer 34. The remainder of the protective layer 34 comprises elementsinterdiffused from the substrate 32. The protective layer 34 ispreferably from about 0.0005 to about 0.004 inches in thickness, butlesser or greater thicknesses are operable although less desirable.

A ceramic layer 38 optionally overlies the protective layer 34. Theceramic layer 34 is preferably yttria-stablized zirconia, which iszirconium oxide containing from about 6 to about 8 weight percent ofyttrium oxide. Other operable ceramic materials may be used as well. Theceramic layer 34 is preferably from about 0.004 inch to about 0.025 inchthick, most preferably from about 0.005 inch to about 0.015 inch thick.(FIG. 2 is not drawn to scale.)

FIG. 3 depicts a method of preparing a coated article such as theturbine blade 20. An article substrate 32 having a free sulfur contentof less than about 1 part per million by weight, preferably less than0.5 parts per million by weight, and most preferably about 0.2 parts permillion by weight, is furnished, numeral 50. The article substratepreferably comprises a nickel-base superalloy such as Rene' N5 alloy,whose composition is set forth above, with the above-discussed sulfurlimitation. The article substrate 32 has the shape of the final desiredcoated article, with the same dimensions reduced only by an allowancefor the thickness, if any, of protective system. In the case depicted inFIG. 1, the article substrate 32 would have the shape of the turbineblade 20. In a typical commercial operation, the base metal of thearticle substrate 32 is melted and then cast into the shape of thearticle substrate by directional solidification to produce an orientedmicrostructure in the article substrate.

As used herein, “free sulfur” is sulfur present in elemental form andnot chemically combined with another element. Free sulfur is to becontrasted with combined sulfur, in which the sulfur is chemicallycombined with one or more other elements. The total sulfur content isthe free sulfur content plus the combined sulfur content. The importantdistinction between free sulfur and combined sulfur, for the presentpurposes, is that free sulfur can diffuse to the surface 36 of thesubstrate 32 and promote the debonding of the protective system 30 fromthe substrate 32. Combined sulfur, on the other hand, is fixed at asingle location and cannot move to the surface of the article substrate.

The article substrate 32 having a free sulfur content below about 1 partper million by weight can be achieved in several ways. In one, onlythose heats of material having such a sulfur content would be selectedand used to fabricate the article substrate. This approach is expectedto have limited usefulness, because most alloys tend to have largeramounts of sulfur present due to their method of melting and casting.

In another and currently preferred approach, a scavenger element thatreacts with free sulfur to produce combined sulfur may be present in themolten base metal which is used to make the article substrate. Thescavenger element must be present in a sufficient amount to reduce theremaining free-sulfur—that sulfur which does not react to producecombined sulfur—to the required low level of less than about 1 part permillion by weight. The scavenger element is preferably calcium, whichreacts with free sulfur to produce calcium sulfide, or magnesium, whichreacts with free sulfur to produce magnesium sulfide.

In another approach, the article substrate is wade of a base metalhaving a free sulfur content of more than about 1 part per million byweight. The article substrate is thereafter processed to reduce itssulfur content to less than about 1 part per million by weight. Thepreferred processing is achieved by contacting the article substrate toa reducing gas at elevated temperature. The reducing gas reduces andreacts with the sulfur at the surface of the article to produce agaseous sulfide which is carried away into the gaseous atmosphere. Anynative oxide present at the surface of the substrate must not act as abarrier to sulfur removal in this process. If any such barrier ispresent, it must be removed prior to the sulfur-removal treatment.Sulfur that is initially within the interior of the article substratediffuses to the surface, and is in turn reduced and reacted to remove itfrom the substrate. The process is continued until the sulfur content ofthe article substrate is reduced to the required level. Desulfurizationis achieved by heating the article substrate 32 in a reducingatmosphere, preferably 1 atmosphere of flowing hydrogen gas or otherhydrogen-containing gas, to a high temperature for a period of timesufficient to react and remove the sulfur from the substrate. Fornickel-base superalloys, the desulfurization heat treatment temperatureis conducted at least about 2200° F., to achieve the required degree ofdesulfurization in a commercially acceptable time. The temperature towhich the substrate is heated will depend upon the composition of thesubstrate. For the preferred Rene' N5 substrate, a temperature of about2250-2340° F. is preferred. For an actual substrate in the form of aturbine blade airfoil, desulfurization was successfully completed byexposure to 1 atmosphere of flowing hydrogen gas at 2336° F. in a timeof 100 hours.

The desulfurization treatment, when used, is preferably conducted duringmelting of the alloy, which is thereafter cast into the desired shape ofthe article substrate. Equivalently for the present purposes, thedesulfurization may be performed after the casting of the base metal toform the article substrate, and just before the deposition of theprotective system 30. If the metal is desulfurized prior to forming thearticle substrate, care must be taken so that sulfur is notre-introduced when the article substrate is formed.

The protective system 30 is formed on the free surface 36 of thesubstrate 32. The protective layer 34 is formed by any operable process.Preferably, a platinum sublayer is first deposited. The deposition isaccomplished by electrodepositing platinum from a platinum-containingsolution onto the substrate. An operable platinum-containing solution isan aqueous solution of 4-20 grams platinum per liter of platinumprovided as Pt(NH₃)₄HPO₄, and the voltage/current source is operated at½ to 10 amperes per square foot of surface being electroplated. Aplatinum sublayer about 5 micrometers thick is deposited in 1-4 hours ata temperature of 190-200° F.

An aluminum sublayer is deposited overlying the platinum sublayer 34.The aluminum sublayer is deposited by any operable approach, with vapordeposition preferred. In that approach, a hydrogen halide gas, such ashydrogen chloride, is contacted with aluminum metal or an aluminum alloyto form the corresponding aluminum halide gas. The aluminum halide gascontacts the platinum sublayer already deposited on the substrate,depositing the aluminum thereon. The deposition occurs at elevatedtemperatures such as from about 1925° F. to about 1975° F. so that thedeposited aluminum atoms interdiffuse into the platinum layer 34 duringa 4 to 20 hour cycle, This technique allows doping or alloying elementsto be deposited into the aluminum sublayer if desired, from the halidegas. During this deposition the aluminum sublayer, the platinumsublayer, and the metal of the substrate interdiffuse to form theplatinum aluminide. Additional interdiffusion may be accomplished ifdesired by maintaining the structure at elevated temperature after theflow of halide gas is discontinued. The platinum and aluminum sublayersinterdiffused with the substrate form the protective layer 34 thatinhibits oxidation and corrosion damage to the airfoil 22, duringexposure at intermediate temperatures.

The ceramic layer 38 is optionally applied overlying and contacting theprotective layer 34, numeral 54 of FIG. 3. The ceramic layer 38 may bedeposited by any operable technique, with electron beam physical vapordeposition (EBPVD) being preferred. Other techniques include, forexample, air plasma spray, low pressure plasma spray, and electron beamphysical vapor deposition.

Although particular embodiments of the invention have been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

1. A coated article, comprising: an article substrate having a surfacecomprising a free sulfur content of less than about 1 part per millionby weight; a protective layer disposed on the surface of the articlesubstrate, the protective layer comprising a platinum aluminidediffusion layer; and wherein the free sulfur content further beingsufficiently low to maintain adherence of the protective layer to thesurface when exposed to gas turbine engine service conditions.
 2. Thecoated article of claim 1, further including a ceramic layer overlyingthe protective coating.
 3. The coated article of claim 2, wherein theceramic layer is yttria-stabilized zirconia.
 4. The coated article ofclaim 1, wherein the coated article is a component of a gas turbineengine.
 5. The coated article of claim 1, wherein the coated article isselected from the group consisting of a turbine blade and a turbinevane.
 6. The coated article of claim 1, wherein the article substratecomprises a nickel-base alloy.
 7. The coated article of claim 1 whereinthe portion of the article substrate has a free sulfur content of morethan 0 but less than about 0.5 parts per million by weight.
 8. A methodfor preparing a coated article, comprising the steps of: furnishing anarticle substrate comprising a nickel-base superalloy, the articlesubstrate having a surface comprising a free sulfur content of less thanabout 1 part per million by weight; forming a protective layer at asurface of the article, the protective layer comprising a platinumaluminide diffusion coating; and wherein the free sulfur content furtherbeing sufficiently low to maintain adherence of the protective layer tothe surface when exposed to gas turbine engine service conditions. 9.The method of claim 8, wherein the step of forming the protective layerfurther includes the steps of: depositing a sublayer of platinumoverlying the surface of the article substrate; depositing a sublayer ofaluminum overlying the sublayer of platinum, and interdiffusing aportion of the substrate, the sublayer of platinum, and the sublayer ofaluminum.
 10. The method of claim 8, wherein the article substrate has ayttrium content of no greater than about 200 parts per million byweight.
 11. The method of claim 8, wherein the article substrate has ayttrium content of less than about 10 parts per million by weight. 12.The method of claim 8, wherein the article substrate has a yttriumcontent of from about 10 to about 200 parts per million by weight. 13.The method of claim 8, including an additional step, after the step offorming, of applying a ceramic layer over the protective layer.
 14. Themethod of claim 13, wherein the ceramic layer is yttria-stabilizedzirconia.
 15. The method of claim 8, wherein the coated article is acomponent of a gas turbine aircraft engine.
 16. The method of claim 8,wherein the coated article is selected from the group consisting of aturbine blade and a turbine vane.
 17. The method of claim 16 , whereinthe coated article is a turbine vane.
 18. The method of claim 16,wherein the portion of the article substrate has a free sulfur contentof more than 0 but less than about 0.5 parts per million by weight.